This project is designed to provide information about the organization of neuronal systems in the mammalian spinal cord that are involved in the neural control of movement, as studied in adult cats in vivo and in the isolated brain stem and spinal cord of neonatal mice studied in vitro. Adult cats are used to examine the organization of specific sets of last-order interneurons in reflex pathways that project directly to motoneurons. Modulation of synaptic potentials produced in motoneurons by input from a variety of afferent systems allows inferences about the convergence onto pathway interneurons of control inputs from primary afferent, supraspinal descending systems, and the spinal central pattern generators (CPGs) that generate rhythmic motoneuron output during fictive locomotion and scratching. Identifiable groups of last-order interneurons receive input from specific low-threshold cutaneous afferents and produce disynaptic EPSPs or IPSPs in specific groups of lumbosacral motoneurons. These disynaptic pathways are powerfully modulated by the spinal central pattern generator (CPG) for locomotion during fictive stepping in decerebrate animals. The specific patterns of CPG modulation in different motoneuron species allow identification of specific sets of segmental interneurons, each with its own patterns of convergence from input control sources. We have found that these patterns of reflex pathway modulation provide important markers for specific states of the locomotor CPG. During FY1997, we compared the control of cutaneous and muscle afferent reflex pathways during fictive locomotion and fictive scratching. To our surprise, we found that oligosynaptic cutaneous pathways from the distal hindpaw, that are markedly facilitated during certain parts of the step cycle in fictive locomotion, are profoundly depressed throughout fictive scratching. In contrast, disynaptic excitation and inhibition produced by group I muscle afferents are powerfully modulated during both fictive locomotion and scratching. In general, disynaptic group I excitation is maximal when the motoneurons are depolarized and active. There was one interesting exception to this pattern. The motoneurons mechanical synergist muscles flexor digitorum longus (FDL) and flexor hallucis longus(FHL) receive mono- and disynaptic group I EPSPs from muscle spindles in either muscle. The disynaptic group I EPSPs in both sets of motoneurons are facilitated during the extension phase of fictive locomotion but during the flexion phase of fictive scratching. The FHL motoneurons are depolarized and active during the extension phase of both movements, while the FDL cells are depolarized and active during the flexion phase. The data suggest that a common set of excitatory last-order interneurons mediates between group I afferents from both muscles and projects to both sets of motoneurons.